SLP tokens have become popular on BCH and on Memo. So far, there has been a single type of SLP token. Last month the SLP team released a spec for a 2nd type of SLP token, Non-Fungible Tokens (NFTs).
With normal SLP tokens you can send any quantity. NFT SLP tokens are "non fungible", meaning each token is unique. Tokens cannot be split or combined. Each token can only be sent by itself.
In the Ethereum world, standard SLP tokens can be thought of similar to ERC-20 tokens. ERC-20 tokens are known to be used for things like ICOs. The new SLP NFT tokens are similar to ERC-721 tokens. ERC-721 tokens are known to be used for crypto collectibles, like CryptoKitties.
NFT child tokens are linked to NFT group tokens. NFT group tokens work like standard SLP tokens in that they can be combined and split. An NFT child token is created by burning a group token.
An example use case for NFT tokens are event tickets. The events would be NFT group tokens and tickets would be NFT child tokens. Since each child token is unique, they can have information like seat number, etc.
Wallets must update to support NFT tokens. Until a wallet has upgraded it is unsafe to send NFT tokens to that wallet, even group tokens. NFT tokens use a different token type and if a wallet does not properly support them they will get burned. It does not look like many wallets have added SLP NFT token support yet.
Start creating tokens on Memo:
For more info about the differences between SLP token types, check out this Bitcoin.com article:https://news.bitcoin.com/slp-developers-publish-specs-for-a-unique-type-of-non-fungible-tokens/
For more info on the SLP spec, see their Github:https://github.com/simpleledger/slp-specifications/blob/master/slp-nft-1.md